1. Field of the invention
The present invention consists in a fiber optic link. The invention also concerns a transmission system comprising a link of the above kind. Finally, the invention concerns a method for pushing back the optical power limitation due to stimulated Raman scattering in an optical fiber link. The invention applies in particular to so-called xe2x80x9crepeaterlessxe2x80x9d links. Such links are distinguished by the fact that electrically active components are used only in the terminal equipments.
2. Description of the Prior Art
Repeaterless fiber optic links have the particular feature of requiring injection of very high optical powers info the optical fiber to achieve great distances. Two types of optical waves are injected into the fiber. The first type is the signal wove at approximately 1 550 nm which is modulated and conveys the information to be transmitted. The second type of optical wave is the so-called pump wave in the wavelength range from 1 400 nm to 1 500 nm and which is a continuous power injected into the optical fiber in order to amplify the signal.
The pump wave can be used in many different ways. The pump wove can be sent from the sending terminal or from the receiving terminal. The scheme most widely used at present consists in placing a section of doped fiber in the link a few tens of kilometers from the terminal from which the pump wave is sent. The doped fiber is activated by the pump optical wave and amplifies the signal. The pump wave can be injected into the same optical fiber as that which carries the signal or into a separate optical fiber. The two techniques con be combined. If the signal and pump waves are on the same fiber, the pump wave amplifies the signal because of stimulated Raman scattering, which is beneficial up to a particular pump power level.
Increasing the range of the link requires using very high signal and pump powers (in the order of one Watt). However, the signal power and the pump power that can be injected are limited by various non-linear effects, in particular the Brillouin effect, the Kerr effect and stimulated Raman scattering. These effects are described in xe2x80x9cNonlinear Fibre Opticsxe2x80x9d, G.P. Agrawal, Academic Press 1980.
Limitation by various non-linear effects is described in xe2x80x9cThe performance limits of unrepeatered systemsxe2x80x9d, A. Hadjifotiou, Suboptic ""93, Mar. 29-Apr. 2, 1993, Versailles, France; with reference to stimulated Raman scattering, this publication simply proposes a threshold value for the sending power corresponding to the value of the sending power for which the power frequency shifted because of stimulated Raman scattering (Stokes power) is equal to the power of the signal at the fiber exit.
A transmission system using Raman pre-amplifiers is described in xe2x80x9cRayleigh scattering limitations in distributed Raman pre-amplifiersxe2x80x9d, P. B. Hansen et al, OFC""97 technical digest, paper FA2, Dallas, February 1997. This publication calculates the limitation imposed by Rayleigh scattering in Raman pre-amplifiers.
The invention is based on the new discovery that the phenomenon limiting the injection of high optical powers is stimulated Raman scattering. For high injected signal or pump powers stimulated Raman scattering produces a very high gain in the fiber which, combined with Rayleigh tire reflections (intrinsic reflections of the fiber), cause power instabilities and oscillations that prevent transmission. This phenomenon also creates noise which is amplified to the detriment of the signal of the pump to be transmitted. This phenomenon is mentioned in the publication by A. Hadjifotiou mentioned above, along with other limitations on link performance; however, that publication merely proposes modeling of the corresponding power limit, does not specify if this is the limiting effect, and does not describe any solution to the limitation problem.
The invention proposes a solution to the problem of limitation of the optical power injected into fiber optic links; in particular, it caters for longer repeaterless links by enabling the injection of higher powers. Compared to currently possible commercial link lengths of 400 km or laboratory link lengths of 500 km, such as those described in xe2x80x9c511 km at 2.5 Gbit/s and 531 km at 622 Mbit/sxe2x80x94Unrepeated Transmission with Remote pumped amplifiers, Forward Error Correction and Dispersion Compensationxe2x80x9d, S. S. Sian et al, the invention enables the length of the link to be extended by more than 80 km. In power terms, the pump powers currently injected are in the order of 1.3 W, as described for example in xe2x80x9cUnrepeated WDM Transmission Experiment with 8 Channels of 10 Gb/s over 352 kmxe2x80x9d, P. B. Hansen et al, IEEE Photonics Technology Letters, vol. 8 No 8, August 1996; the invention enables injection of pump powers up to 10 W.
To be more precise, the invention proposes an optical fiber link including attenuator means in the wavelength region in which the sent signal creates the Raman gain.
The attenuator means are advantageously bidirectional.
They preferably also attenuate Rayleigh reflections in the wavelength region in which the sent signal creates the Raman gain.
In one embodiment, the attenuator means induce reflections in the link at a level less than xe2x88x9220 dB, preferably less than xe2x88x9240 dB at the maximum Raman gain wavelength.
In another embodiment, the attenuator means attenuate the maximum Raman gain wavelength at least 10 dB.
The attenuator means advantageously attenuate at a level less than 1 dB in the wavelength region of the sent signal.
The attenuator means can comprise one or more of the following: a section of optical fiber doped with materials that are more absorbent in the wavelength region in which the sent signal creates the Raman gain than in the wavelength region to be transmitted; filters attenuating in the wavelength region in which the sent signal creates the Raman gain; optical isolators.
In this case, these materials include rare earths such as terbium or dysprosium, for example. The concentration of rare earth in the fiber section is advantageously greater than 0.01 ppm.
In one embodiment, the attenuation per unit length in said fiber sections at the wavelength at which the sent signal creates the Raman gain is at least two times greater than the attenuation at the wavelength of the sent signal.
Localized attenuator means can also be provided at at least one point of the link, the total length of the attenuator means being less than 10% of the total length of the link.
Distributed attenuator means can be provided in the link, the total length of the attenuator means being in the range 10% to 100% of the total length of the link.
In one embodiment, the attenuator means in the Raman gain region are placed at locations of the link such that at any point of the link the cumulative Raman gain value from the end of the link or from other attenuator means is less than a limit value.
The limit value preferably depends on reflections in the link.
In another embodiment, attenuator means in the Raman gain region are used to transmit high transmission signal wave powers in the wavelength range 1 520 nm to 1 580 nm or high pump wave powers in the wavelength range 1 400 nm to 1 500 nm.
A plurality of attenuator means can be provided.
The invention also concerns a transmission system comprising at least one such link.
The invention finally concerns a method of transmitting high-power optical signals in a fiber optic link including at least one step of attenuation in the region of the wavelength at which the sent signal creates the Raman gain.
In one embodiment, the attenuation step comprises attenuation in both propagation directions on the link.
A step of attenuating Rayleigh reflections in the wavelength region in which the sent signal creates the Raman gain can also be provided.
The attenuation step advantageously comprises an attenuation of at least 10 dB at the maximum Raman gain wavelength.
The attenuation step preferably induces an attenuation at a level less than 1 dB in the wavelength region of the sent signal.
In one embodiment, the attenuation step comprises one or more of the following steps: propagation through sections of optical fiber doped with materials absorbing more in the wavelength region in which the sent signal creates the Raman gain than in the region of the wavelength to be transmitted; filtering using attenuating filters in the wavelength region in which the sent signal creates the Raman gain; reflection by optical isolators.
In the above cases the materials can include rare earths such as terbium or dysprosium. The rare earth concentration in the fiber section is then preferably greater than 0.01 ppm.
In one embodiment, the attenuation per unit length in said fiber sections at the wavelength at which the sent signal creates the Raman gain is at least twice the attenuation at the wavelength of the sent signal.
The attenuation step can be effected in a localized manner at at least one point of the link or in a distributed manner along the link, over a length in the range 10% to 100% of the total length of the link.
The attenuation step is advantageously effected in the link so that at any point of the link the cumulative Raman gain value is less than a limit value. The limit value is preferably dependent on reflections in the link.
A plurality of attenuation steps is advantageously provided.
Other features and advantages of the invention will become apparent on reading the following description of various embodiments of the invention given by way of example only and with reference to the drawings.